How to understand it
Plus and minus signs distinguish converging and diverging power. Cylinder axis uses angular degrees, a different unit.
An example
For a simple +2.00 D lens, the model focal length is 0.50 m. This is not a method for prescribing someone’s glasses.
A common mix-up
0.50 D, an axis of 50° and visual acuity 0.5 describe three different things.
See the idea in a full guide →
Sources & verification
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